Highly stable integrated time reference

Abstract

An integrated oscillator that may be used as a time clock includes circuitry that oscillates about an RC time constant, which RC time constant is adjustable to provide a desired frequency of oscillation. More specifically, the oscillator includes a capacitor array that has a plurality of capacitors coupled in parallel wherein each capacitor may be selectively included into the RC time constant or selectively excluded there from. Rather than setting the capacitance values to a desired capacitance value, a system for adjusting the time constant includes circuitry for measuring an output frequency and for comparing that to a certified frequency source wherein the time constant is adjusted by adding or removing capacitors from the capacitor array until the frequency of the internal clock matches an expected frequency.

What is claimed is:  
     
       1. An integrated clock system, comprising: 
       a reference voltage;  
       a transconductor element coupled to receive a reference voltage from the reference voltage source;  
       a capacitive element coupled to receive a proportional voltage output from the transconductor element;  
       the capacitance circuit comprising a plurality of capacitors, each of which is selectable; and  
       a comparator coupled to receive the reference voltage from the voltage reference source and also coupled to receive a voltage that is across the terminals of the capacitor circuitry wherein the comparator generates a reset signal to cause a capacitor circuitry to be discharged or reset whenever the voltage across the capacitor circuitry matches the reference voltage.  
     
     
       2. The system of  claim 1  wherein the comparator circuit includes a resistive element on an input side that is coupled to receive the reference voltage. 
     
     
       3. The system of  claim 1  wherein the capacitor circuitry is coupled in parallel to the transconductor element. 
     
     
       4. The system of  claim 1  wherein the capacitor circuit comprises more than ten capacitors. 
     
     
       5. The system of  claim 1  wherein the capacitor circuit comprises at least thirty-two capacitors. 
     
     
       6. The system of  claim 1  wherein each capacitor is coupled in series with a switch, which switch is controlled by an external controller to switch the corresponding capacitor in or out of the circuit. 
     
     
       7. The system of  claim 1  wherein the capacitor circuit includes exactly thirty-two capacitors.